Oxygen isotope ratios of contemporary waters and diatom silica: implications for palaeoclimate research.

Despite the increasingly frequent use of oxygen isotopes in diatom silica as a proxy for past environmental change, the processes which control these isotope ratios remain poorly understood. Various factors affect the oxygen isotope composition of sedimentary diatom silica, beginning with the atmospheric changes that determine the isotope composition of precipitation through the controls over assimilation of silica within the diatom cell to the processes affecting silica sedimentation and diagenesis. An understanding of these contemporary processes is an important prerequisite for the use of lake sediment records as archives of past climate change. The aim of this thesis was to investigate the cascade of factors which determine the oxygen isotope composition of sedimentary diatom silica, using a case study of Lochnagar, a remote upland lake in Scotland. The climate-lake interface at Lochnagar was studied using bi-weekly monitoring of the oxygen isotope composition of precipitation (6180p) and lake water (6180L) (between December 1999 and September 2005) and monthly monitoring of the cell density and oxygen isotope composition of living diatoms (5180Siiica) (between May 2004 and November 2005, excluding periods of lake ice cover). Three supplementary datasets of monthly 5180p from sites in Great Britain and Eire (Wallingford, Oxfordshire Keyworth, Nottinghamshire and Valentia, Kerry) were also analysed to obtain a wider geographic perspective. 5180Siiica was analysed from the surface sediments of a series of lakes across Europe in order to assess the relationship between climate and 5180Sinca over a large continental gradient. The research demonstrates considerable complexity in the climate-lake-sediment interface at Lochnagar. 6180p is a sensitive climate tracer, reflecting changes in air temperature and precipitation amount, however the response varies with site location. Changes in S18Ol at Lochnagar reflected changes in 6180p, but the response was mediated by limnological processes, primarily the water throughflow rate, which is a function of precipitation amount. Diatom abundance varied markedly over the annual cycle, with peak abundance during Spring, leading to a probable seasonal bias in the sediment 6180Siiica record. 6180SiiiCa values also fluctuated markedly throughout the seasonal cycle, although no systematic relationship with potential physical controls was observed. However, 5180SiiiCa values in the surface sediment dataset do correlate with the continental climate gradient, suggesting that diagenetic effects play an important role in fixing the climate signal. The thesis concludes that potential does exist for the use of 5180SitiCa in palaeoclimate research, despite considerable complexity within the sediment-climate interface. The processes which combine to determine the sedimentary 5180Siiica signal are combined in a basic holistic model to enable a more rigorous approach to interpreting the 5180Siiica record from Lochnagar. However, further detailed research into controls over isotope fractionation during silica assimilation by diatoms, and subsequent diagenesis, is required if 5180Stiica records are to be confidently used to reconstruct past climate